Page 24 - Landscape-Issue141(2)

IRRIGATION FEATURE

UNDERSTANDING WATER

DISTRIBUTION IN SOIL:

DRIPPER SPACING AND DELIVERY RATE

By Jovan Erasmus, Agronomist, Netafim SA

In the precision planning process of a drip irrigation system, one of the crucial steps
is to determine the correct delivery rate and dripper spacing that will be required

for optimal irrigation for your conditions.

Each type of soil has a unique texture and structure

o determine the required delivery horizontally by capillary action. Hydraulic and the water will drain past. Soil with
rate and dripper spacing, we need conductivity is the water’s potential to larger particles has lower porosity and a
Tto understand the way water is move in soil. Water moves on a gradient lower surface to volume ratio. Clay type
distributed in soil and what affects water from wetter to dryer soil, and the wetter the soils with smaller soil particles have
distribution in soil. Each type of soil soil, the easier water will move through it, higher porosity with a higher surface to
has a unique texture and structure and as conductivity is higher. If soil is dryer, the volume ratio. More porous soils allow
these physical characteristics affect soil empty pores in the soil must be filled first, better water-air balance management in
infiltrability, hydraulic conductivity, water and water distribution is slower. the soil and create space for the water to
retention and porosity. All of these factors move through the soil.
represent important processes that work Water retention
together holistically to determine the Water retention is the soil’s ability to hold Saturated circles
movement of water in soil. water and nutrients. This greatly depends When crops are drip irrigated, the water
on the size of the soil particles. Soils with is delivered in one spot, and we rely on
Infiltrability smaller particle sizes offer better water the soil’s physical ability to distribute
Soil infiltrability is the rate at which water retention. Sandier soils generally have the water. A wetted bulb or saturated
can be absorbed into the soil and is mostly larger particle sizes compared to heavier circle forms under the dripper. Your soil’s
determined by soil texture or the percentage clay soils, which is why clay soils have physical characteristics and the delivery
of sand, silt and clay in soil. It is important that better water retention than sandier soils. rate of the dripper will determine the
the delivery rate of your irrigation system is Better water retention simply means that dimensions of this circle.
balanced with the soil’s infiltration rate. If the you can put more water into the soil and Roots require oxygen to grow and take
water is delivered at a rate higher than the that it will hold the water for longer. up water and nutrients, and it is therefore
rate at which it is absorbed into the soil, there important that there is balance between
tends to be greater horizontal spread of water Porosity the water and air that fills soil pores. As
close to the soil surface. This brings risks such Soil’s porosity depends on the spaces water infiltrates soil, it replaces air in the
as forming of the shallow root-zone, losing between soil particles: the macro and pores. In drip irrigation, this creates a
water off the ridges in orchards, ponding, run- micro pores. These are the spaces where gradient between low and high oxygen
off and higher evaporation. In short, water- water and nutrients are stored and where levels in the soil. Drippers with higher
use efficiency will be negatively affected. the roots will grow. delivery rates displace more air faster,
Macro pores allow easy movement creating variation in the soil’s oxygen
Hydraulic conductivity of water and air through soil. Water will levels and leading to lower uptake of
Water moves through soil as it is infiltrate the soil until these pores are water. Drippers with lower delivery rates
pulled down by gravity and distributed filled, at which point it reaches saturation allow for better hydraulic conductivity

22 Landscape SA • Issue 141 2024 Check us out www.salandscape.co.za